The present invention relates to a zoom lens unit used for a television camera, a video camera, or a digital camera, specifically to a zoom lens unit for a monitoring system as well as an imaging apparatus with the zoom lens unit.
In recent years, a market of imaging devices (e.g., cameras) used for a security usage, etc. has grown, and requests or/and requirements from users are being various and diversified. Yet, most of the users demand a camera with high image quality (resolution) and high magnification ratio, and it is highly important to achieve both of the high image resolution (high image resolving power) and high magnification ratio for zoom lens units used as photographing lenses.
To achieve the high image resolution, the zoom lens unit should have a resolving power compatible with the image pickup element of at least 1,000,000 to 5,000,000 pixels in the entire zooming range. Further, for the high magnification ratio, the zoom lens unit should have a magnification ratio of approximately 16 times.
Additionally, it is desirable that aberrations of the lens are sufficiently corrected in the near infrared region (i.e., in a range where the light wavelength is equal to or shorter than around 900 nm), thereby enabling photographing in a dark place.
Conventionally, several zoom lens units have been developed. For example, Japanese Laid-Open Patent Applications No. 1985(Sho60)-126618 (Document 1), No. 1986(Sho61)-204610 (Document 2), and No. 1992(Hei04)-88310 (Document 3) teach zoom lens units that include, in order from an object side to an image side, a first lens group with positive refractive power, a second lens group with negative refracting power, a third lens group with negative refracting power, and a fourth lens group with positive refractive power having an anomalous dispersion glass; and the first lens group includes, in order from the object side to the image plane side, a cemented lens composed of a negative lens and a positive lens, a positive lens, and another positive lens.
The zoom lens units disclosed in Documents 1 to 3 may achieve high magnification ratio, large aperture (large diameter), and temperature resistance. In contrast, those Documents keep silent regarding the aberrations correction in the near infrared region.